With Invisibility, a creature is visually undetectable. Invisibility is categorized as a condition.
Invisibility doesn't show up in the Skills Tab under Hide as it isn't a bonus to Hide. You're simply considered to be hidden with a Hide score of 20.
As with Hide, an invisible creature that stands too long within an enemy's visual arc has a strong chance of being detected. The more light in the area, the greater the likelihood.
An invisible creature in the water displaces water, revealing its location.
Being invisible does not make a creature soundless. Without using Sneak while moving, the creature will be heard with a Listen check by enemies as if your Move Silently skill were zero.
If an Invisible creature attacks a target or uses an object (Door, lever, etc.), that creature loses invisibility.
Ethereal creatures are Invisible.